1. Field of the Invention
The present invention pertains generally to methods used in nuclear magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR), and more specifically to NMR, MRI, or spectroscopic MRI in inhomogeneous magnetic fields.
2. Description of the Relevant Art
What follows is a brief introduction to some NMR basics, with the intention of describing some of the terminology used in this disclosure.
Magnetic resonance spectroscopy and imaging require that a sample be subjected to a static magnetic field {right arrow over (B)}0. Units of field strength are in Tesla (T), where 1T is 104 Gauss. Typically the static field is produced by a superconducting magnet, or, as in the case of portable systems, by permanent magnets or electromagnets. Nuclei that possess spin (this is an intrinsic property like charge and mass, and one can think of it as an intrinsic angular momentum) can be studied with NMR (e.g. 1H, 13C, 19F), though most people are familiar with (hydrogen nucleus 1H) proton MR. In the absence of a field, nuclear magnetic moments (nuclear magnetic moment is associated with spin) are random because all allowed orientations have the same energy. When the sample is placed in the strong magnetic field, a macroscopic magnetization parallel to the field appears.
The second requirement for MR experiments is to excite the sample. This is done by a second field, {right arrow over (B)}1 perpendicular to the static field {right arrow over (B)}0. This {right arrow over (B)}1 field is generally produced by sending radio frequency (or RF) current through a coil, most typically a solenoid or a saddle coil and producing a magnetic field {right arrow over (B)}1, which oscillates at the Larmor frequency (ω0=γB0), where gamma is the gyromagnetic ratio which is characteristic of the nucleus and has units of Frequency/field strength). For this reason in NMR, people prefer using frequency units when referring to field strength magnitude. For example, a 2.3 T magnet would be referred to as a 100 MHz magnet, because 100 MHz is the resonance frequency of the 1H hydrogen nucleus in the reference compound (TMS) in that field.
One needs to have a homogeneous static magnetic field over the sample, so that it is easy to determine the microenvironment. If the field is inhomogeneous (i.e. {right arrow over (B)}0({right arrow over (x)}) depends on position in space), local environment effects are negligible relative to {right arrow over (B)}0({right arrow over (x)}) variations.
The local environment depends on the electronic cloud surrounding the nucleus, and the effect is called shielding, s (imagine that the electrons “protect” the nucleus from the static field). Chemical shift is the NMR chemical signature of a substance, which is the variation of the resonance frequency because of shielding Ω0=γB0(1−σ). NMR spectroscopists like to measure chemical shifts, as the difference from a standard, Δω=γB0Δσ=γB0(σref−σ) in units of parts per million (ppm). Sometimes inhomogeneities or gradients (variations of field for variations of space) in a magnet are measured in ppms, since this is the scale of most importance for NMR measurements.
Traditional measurements of samples using the physical process of magnetic resonance, both nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) utilize static magnetic {right arrow over (B)}0 fields of high uniformity, typically maintaining homogeneity of 10-100 parts per billion (ppb) within a sampling volume intended to be tested.
U.S. Pat. No. 6,674,282 describes a method and apparatus for ex-situ nuclear magnetic resonance spectroscopy for use on samples outside the physical limits of the magnets in inhomogeneous static and radio-frequency fields, which is hereby incorporated by reference in its entirety. Chemical shift spectra can be resolved with the method using sequences of correlated, composite z-rotation pulses in the presence of spatially matched static and radio frequency field gradients producing nutation echoes. The amplitude of the echoes is modulated by the chemical shift interaction and an inhomogeneity free FID (free induction decay) may be recovered by stroboscopically sampling the maxima of the echoes. In an alternative embodiment, full-passage adiabatic pulses are consecutively applied. One embodiment of the apparatus generates a static magnetic field that has a variable saddle point.
Additionally, and by way of background, U.S. Pat. Nos. 4,968,939, 6,426,058, 6,818,202, 6,159,444, 6,652,833, and 6,885,192 are also incorporated by reference in their entireties.